The present invention relates to a hydraulic control system for an automatic transmission, capable of reducing shocks in shifting the automatic transmission.
A hydraulically controlled automatic transmission is shifted by applying oil pressure to a frictionally engaging device to which oil pressure has not been applied or by releasing oil pressure from a frictionally engaging device to which oil pressure has been applied. Accordingly, the transitional characteristics of oil pressure in controlling frictionally engaging devices for shifting the automatic transmission, particularly those in which oil pressure is applied to the frictionally engaging devices, are very important for reducing shocks in shifting the automatic transmission.
Conventional hydraulic control systems for automatic transmissions have an accumulator in the form of a hydraulic cylinder including a piston for regulating the transitional characteristics of oil pressure to be applied to the frictionally engaging device. The accumulator is provided between a shift valve for changing the oil supply passage to change the gear stage of the automatic transmission, and the frictionally engaging device. This accumulator maintains the supply oil pressure to be applied to the frictionally engaging device substantially at a set oil pressure for the period of time during which the piston slides within the cylinder to reduce shocks attributable to the variation of the input system in rotating speed in shifting the automatic transmission.
The optimum value of the set oil pressure is dependent on the torque of the engine transmitted to the automatic transmission. The set oil pressure can be controlled by regulating the oil pressure in the backpressure chamber of the accumulator.
Japanese Patent Laid-Open (Kokai) No. 56-138533 discloses a hydraulic control system in which oil pressure which is varied according to the throttle opening (the degree of opening of the throttle valve of the engine: which represents engine torque), such as the line pressure or the throttle modulator pressure, is applied to the backpressure chamber of such an accumulator to regulate the transitional oil pressure applied to the frictionally engaging device according to the torque of the engine by the accumulator.
Japanese Patent Laid-Open (Kokai) No. 61-149657 discloses a hydraulic control system for an automatic transmission, in which the oil pressure applied to the backpressure chamber of an accumulator is adjusted to a further optimum value on the basis of factors such as oil temperature in the automatic transmission, the temperature of the intake air, the type of shifting, the running speed of the vehicle, the engine speed and the supercharging pressure, in addition to the torque of the engine (the throttle opening), to regulate the oil pressure applied to the frictionally engaging device for optimum transitional characteristics. The hydraulic control system disclosed in Japanese Patent Laid-Open (Kokai) No. 61-149657 controls the transitional oil pressure acting on the frictionally engaging device on the basis of the present running condition of the vehicle or various parameters including those representive of the ambient conditions of the vehicle, and hence the hydraulic control system is able to control the transitional characteristics of the oil pressure acting on the frictionally engaging device more minutely.
However, the following problems arise in this hydraulic control system when mounted on a vehicle for practical control operation.
The first problem is due to inaccuracies in oil pressure generating devices. Generally, in controlling the oil pressure acting on the backpressure chamber of the accumulator on the basis of many parameters, basic detection signals provided by detectors are processed by a computer. The computer adjusts the oil pressure to a target oil pressure, for example, through the duty control of a solenoid valve. However, even if the computer provides an appropriate and accurate control signal, the oil pressure will not be appropriately regulated because of the operating inaccuracy of the solenoid valve.
The second problem is the reliability of the fail-safe arrangement. Regulating means operated by an electrical control signal function normally when the detection signals provided by the detectors are processed normally. However, it is possible that an abnormal or faulty signal (representing a condition entirely different from an actual condition) may be applied to the computer when a fault occurs in the electric circuit. Furthermore, when a disconnection or short circuit occurs in a transmission line for transmitting a signal, produced through processing the detection signals provided by the detectors, for controlling the solenoid valve, the signal for regulating the transitional characteristics of the oil pressure applied to the frictionally engaging devices is invalidated, thereby incorrectly enhancing the shocks during shifting of the automatic transmission or, worse yet, preventing shifting of the automatic transmission.
The third problem is caused by a number of varying factors. Although many parameters representing of the running condition of the vehicle and the ambient conditions of the vehicle are employed for controlling the transitional characteristics of the oil pressure, the hydraulic control system inevitably includes causes of indefinite variation which cannot be expected at the stage of design and the causes can develop significantly. For example, the line pressure, namely, the basic pressure, of a hydraulic control system is regulated, in general, on the basis of data obtained through a throttle cable interlocked with the throttle valve of the engine, and a throttle cam. In such an arrangement, the adjustment of the throttle cable is different between vehicles and such difference cannot be taken into consideration in producing a control program to be executed by the computer. Moreover, it is impossible to design the hydraulic control system so as to be able to accomodate variations in the mode of flow of oil in the oil passage due to the accumulation of impurities in the oil. Naturally, these inherent variations which occur in the manufacturing process or develop with time, cause the faulty regulation of the transitional characteristics of the oil pressure applied to the frictionally engaging device, thereby deteriorating the shift characteristics of the automatic transmission.